1. Field of the Invention
The present invention relates to an exposure method, an exposure device and a device producing method which are suitable, in particular, for proximity-type exposure systems for semiconductor device manufacture, in which exposure is effected while the mask and the wafer are kept so close to each other as to leave a minute gap between them.
2. Description of the Related Art
To cope with the present-day further miniaturization of semiconductor devices, exposure devices using a short-wavelength exposure beam, such as X-rays, are being developed. In such exposure devices, a so-called proximity exposure system is generally adopted, in which exposure is effected with the mask and the wafer being kept so close to each other as to leave a minute gap, for example, on the order of several tens of .mu.m between them.
FIG. 25 is a schematic diagram showing an exposure device using X-rays such as synchrotron orbital radiation. The exposure device includes a wafer stage 110 and a mask stage 120. The wafer stage 110 includes a guide device 112 fixed to a wafer stage base 111, a rough movement stage 113 guided in the direction of two axes (the X-axis and the Y-axis) orthogonal to the optical axis (the Z-axis) of the X-rays, and a fine movement stage 114 supported by the rough movement stage 113. The fine movement stage 114 supports a wafer chuck 115 for chucking a wafer Wo and is capable of moving, for example, in six-axis directions (X, Y, and Z-axis directions and .omega.X, .omega.Y, and .omega.Z-axis directions) by a minute amount on the rough movement stage 113. The fine movement stage 114 is moved in the X- and Y-axis directions to thereby effect final alignment of the mask Mo with respect to the wafer Wo. At the same time, an exposure gap setting movement is effected, in which the fine movement stage 114 is moved in the Z-axis direction to bring the wafer Wo and the mask Mo close to each other so as to leave a predetermined minute gap (exposure gap) between them. The mask Mo has a membrane Eo equipped with a pattern (not shown) and a mask frame Ho supporting it. The thickness of the membrane Eo is approximately 2 .mu.m, and the mask frame Ho is chucked by a mask chuck 122 supported by a mask stage base 121 to thereby align the mask Mo with respect to the optical path of the X-rays. Further, the wafer stage base 111 and the mask stage base 121 are fixed to the inner wall of an exposure chamber 101 maintained in a reduced pressure atmosphere of helium gas or the like. In a side wall of the exposure chamber 101, a window opening on a high-vacuum beam duct 102 is provided, in which window there is provided a thin film 103 of beryllium or the like serving as an interception between the atmosphere of the exposure chamber 101 and that of the beam duct 102.
In the proximity exposure system, a so-called step-and-repeat method is generally adopted, according to which a plurality of angles of view of the wafer Wo are sequentially moved stepwise relative to the application area of the X-rays, i.e., a position opposite to the mask Mo, to thereby effect exposure. The exposure gap between the mask Mo and the wafer Wo at the time of exposure is set at approximately 10 to 50 .mu.m. The exposure cycle by the step-and-repeat method is conducted as follows:
When the exposure gap between the wafer Wo and the mask Mo is set, as described above, to be approximately 10 to 50 .mu.m, there is a concern that the wafer will interfere with the mask Mo during the step movement of the wafer Wo, so that the wafer Wo is retracted in the Z-axis direction to a predetermined retracted position (for example, to a position where the distance (gap) between the wafer and the mask Mo is approximately 100 .mu.m), and then, the rough movement stage 113 is driven and the wafer Wo is moved stepwise to move the next exposure angle of view to a position opposite to the mask Mo. Then, the fine movement stage 114 is driven in the Z-axis direction to bring the wafer Wo and the mask Mo close to each other to set the exposure gap as described above, and the fine movement stage 114 is driven in the X and Y-axis directions to effect final alignment between the wafer Wo and the mask Mo. After this, X-rays are applied to project the pattern of the mask Mo onto the wafer Wo, to transfer and print the mask pattern onto the wafer. After the transfer and printing have been completed, the fine movement stage 114 is reversely moved in the Z-axis direction to retract the wafer Wo to the retracted position. Then, as described above, the rough movement stage 113 is driven to perform step movement by means of which the next exposure angle of view of the wafer Wo is brought to the position opposite to the mask Mo, and, in the same manner as described above, the processes of setting the exposure gap, final alignment, and transfer and printing are repeated.
However, in the above-described conventional technique, the thickness of the membrane of the mask is as thin as approximately 2 .mu.m, which indicates a very low level of rigidity. Thus, when the fine movement stage is driven in the Z-axis direction to set the exposure gap between the mask and the wafer, the membrane may flap to become bent (deformed), with the result that distortion, misalignment, etc., are generated in the pattern of the mask. When exposure is started before such deformation of the membrane has subsided to a sufficient degree, a deterioration in transfer accuracy is generated, so that a long stand-by period is necessary before starting exposure, resulting in a reduction in throughput. Further, such bending of the membrane also occurs when the wafer is retracted to the retracted position after the completion of the exposure. When the amount of deformation is large, various problems are entailed. For example, the membrane and the wafer are brought into contact with each other, or the membrane suffers damage.